In Middle English it had the meaning of "a small child of either sex". Then the word underwent the process of transference based on contiguity and developed the meaning of "a small child of the female sex", so that the range of meaning was somewhat narrowed. In its further semantic development the word gradually broadened its range of meaning. At first it came to denote not only a female child but, also, a young unmarried woman, later, any young woman, and in modern colloquial English it is practically synonymous to the noun woman (e. g. The old girl must be at least seventy), so that its range of meaning is quite broad.
The history of the noun lady somewhat resembles that of girl. In Old English the word (ОЕ hlæfdiZe) denoted the mistress of the house, i. e. any married woman. Later, a new meaning developed which was much narrower in range: "the wife or daughter of a baronet" (aristocratic title). In Modern English the word lady can be applied to any woman, so that its range of meaning is even broader than that of the OE hlæfdiZe. In Modern English the difference between girl and lady in the meaning of woman is that the first is used in colloquial style and sounds familiar whereas the second is more formal and polite. Here are some more examples of narrowing of meaning:
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It should be pointed out once more that in all these words the second meaning developed through transference based on contiguity, and that when we speak of them as examples of narrowing of meaning we simply imply that the range of the second meaning is more narrow than that of the original meaning.